Nodes issue, shadow catcher is visible
What I've realized, after watching the tutorial, putting this together myself, and then looking at your node setup again, is that by disconnecting ID Mask: Index 1 you have broken your node setup. And with the Mix Node's Factor set to 1 there is no meaning in having a Mix Node at all (combining 0% of one image with 100% of another). Sliding it to anything less than 1 won't help either, because it needs the ID Mask to mask your car object back in.

If I were you, I would revisit your original problem with eliminating the jagged edges instead of trying to hack the node setup.